Original bug ID: 5911
Reporter: @mmottl
Assigned to: @garrigue
Status: closed (set by @xavierleroy on 2015-12-11T18:18:27Z)
Resolution: fixed
Priority: normal
Severity: major
Version: 4.00.1
Fixed in version: 4.00.2+dev
Category: typing
Monitored by: @mmottl

Bug description

The following code snippet shows signatures "S" and "T" and two functors "Good" and "Bad":

The functor "Good" substitutes a type in its argument of signature "S" and then accesses a value having this type, proving that the substitution has worked.

The functor "Bad" uses an argument of signature "T", which merely contains a submodule of signature "S", performing the exact same substitution otherwise. This time the substitution fails. Running "ocaml" on this snippet yields:

File "bug.ml", line 13, characters 11-16:
Error: This expression has type t but an expression was expected of type unit

Here is the snippet:

module type S = sig
type t
val x : t
end

module Good (X : S with type t := unit) = struct
let () = X.x
end

module type T = sig module M : S end

module Bad (X : T with type M.t := unit) = struct
let () = X.M.x
end